You can get more information by applying a patch below (and also tests/json/Makefile.in):

diff --git a/tests/json/Makefile.am b/tests/json/Makefile.am
index 90fba79e..7523bb6b 100644
--- a/tests/json/Makefile.am
+++ b/tests/json/Makefile.am
@@ -106,6 +106,8 @@ gpg-agent.conf:
 # a key from a smartcard reader (error might be: Unusable secret key)
 	echo pinentry-program $(abs_srcdir)/../gpg/pinentry > ./gpg-agent.conf
 	echo disable-scdaemon >> ./gpg-agent.conf
+	echo debug-all >> ./gpg-agent.conf
+	echo log-file /tmp/gpg-agent-logfile.log >> ./gpg-agent.conf

T4820 is not related (it's a failure of t-keylist-secret in t-json), while this is failure of t-decrypt.

It looks like computation for NIST P-256 failed on hppa (32-bit big-endian, actually running on 64-bit machine, IIUC).
powerpc is similar (32-bit big-endian, actually running on 64-bit machine), but no failures.

Oh wow. It seems you have already coded the feature request. I didn't want to generate work for you and offered to submit a patch. Not that I am complaining.;-) Thank you!

cc @jukivili @gniibe

I think we should separate this into two tasks:

For better action names see T6420: Kleopatra: improve layout of and text in smartcard management view.

Arghh, a GPGME_DEBUG=3 which shows basic I/O preparation does not exhibit the bug.

Fixing gpg is easy but there is some bug lingering in gpgme which might be a recent regression. An strace shows

Those options where originally intended for debugging but your suggestion makes sense. I also add this to most other tools.

At the failed attempt I notice a DEVINFO_STATUS removal soon after the SWITCHCARD. This is related to the reader status file and triggered by ccid_slot_status. --debug ipc.app,reader might be a better selection of debug flags here.

For VSD there should at least no info message be shown - which has to be clicked away - when keyserver is set to "None"

While we see issues with gpg-agent not starting in different circumstances (like with the empty keylist) I would like to keep this task here about the smart card tab.

Thanks for the rort